diff --git a/agenthub/langchains_agent/__init__.py b/agenthub/langchains_agent/__init__.py
index 66fb268366..f504b234c4 100644
--- a/agenthub/langchains_agent/__init__.py
+++ b/agenthub/langchains_agent/__init__.py
@@ -136,7 +136,7 @@ class LangchainsAgent(Agent):
                 else:
                     d = {"action": "output", "args": {"output": obs.content}}
             else:
-                raise ValueError(f"Unknown observation type: {obs}")
+                d = {"action": "output", "args": {"output": obs.content}}
             if d is not None:
                 self._add_event(d)
 
diff --git a/opendevin/controller/__init__.py b/opendevin/controller/__init__.py
index 787c31cceb..62b3cb9337 100644
--- a/opendevin/controller/__init__.py
+++ b/opendevin/controller/__init__.py
@@ -1,5 +1,6 @@
 import asyncio
 from typing import List, Callable, Tuple
+import traceback
 
 from opendevin.state import State
 from opendevin.agent import Agent
@@ -45,8 +46,12 @@ class AgentController:
         self.state_updated_info = []
         return state
 
-    def add_observation(self, observation: Observation):
-        self.state_updated_info.append((NullAction(), observation))
+    def add_history(self, action: Action, observation: Observation):
+        if not isinstance(action, Action):
+            raise ValueError("action must be an instance of Action")
+        if not isinstance(observation, Observation):
+            raise ValueError("observation must be an instance of Observation")
+        self.state_updated_info.append((action, observation))
 
     async def start_loop(self, task_instruction: str):
         finished = False
@@ -56,6 +61,7 @@ class AgentController:
                 finished = await self.step(i)
             except Exception as e:
                 print("Error in loop", e, flush=True)
+                traceback.print_exc()
                 break
             if finished:
                 break
@@ -67,7 +73,7 @@ class AgentController:
         print("STEP", i, flush=True)
         log_obs = self.command_manager.get_background_obs()
         for obs in log_obs:
-            self.add_observation(obs)
+            self.add_history(NullAction(), obs)
             await self._run_callbacks(obs)
             print_with_indent("\nBACKGROUND LOG:\n%s" % obs)
 
@@ -91,7 +97,7 @@ class AgentController:
         else:
             observation = NullObservation("")
         print_with_indent("\nOBSERVATION:\n%s" % observation)
-        self.state_updated_info.append((action, observation))
+        self.add_history(action, observation)
         await self._run_callbacks(observation)
 
 
diff --git a/opendevin/server/session.py b/opendevin/server/session.py
index bbe44b8a5a..ca49f3f646 100644
--- a/opendevin/server/session.py
+++ b/opendevin/server/session.py
@@ -6,6 +6,7 @@ from fastapi import WebSocketDisconnect
 
 from opendevin.action import (
     Action,
+    NullAction,
     CmdRunAction,
     CmdKillAction,
     BrowseURLAction,
@@ -99,7 +100,7 @@ class Session:
                         await self.send_error("No agent started. Please wait a second...")
 
                     elif event["action"] == "chat":
-                        self.controller.add_observation(UserMessageObservation(event["message"]))
+                        self.controller.add_history(NullAction(), UserMessageObservation(event["message"]))
                     else:
                         # TODO: we only need to implement user message for now
                         # since even Devin does not support having the user taking other